In triangular indexable cutting plates of this kind, part of an edge of the indexable cutting plate forms a main cutting edge in each case. As a rule, in precision turning this main cutting edge extends radially with respect to the workpiece. That part of the adjacent edge which adjoins the corner of the main cutting edge which is in use then forms an auxiliary cutting edge in each case, while the remaining part of this adjacent edge is also again a main cutting edge, which, however, is not in use. Only when a main cutting edge is worn out is the indexable cutting plate released from its holder, turned through 120.degree. and then again inserted in the recess provided for it in the holder, so that now a part of another edge of the triangle forms the main cutting edge. In known indexable cutting plates of this kind, a radius of about 0.4 to 0.8 mm is provided in each case at the transition from the main cutting edge to the auxiliary cutting edge. The centre of this radius is at a distance from the main cutting edge and the auxiliary cutting edge which corresponds in each case to the radius itself, so that the radius passes over uniformly into the main cutting edge and the auxiliary cutting edge. A small radius of this kind at the transition from the main to the auxiliary cutting edge has the disadvantage that only a limited feed can be operated in precision turning, because otherwise the machined surface shows fine grooves which are frequently undesirable. The slow feed results in longer machining times and thereby also in higher machining costs. Moreover, the cutting tip with a relatively small radius of curvature is highly sensitive.
In indexable cutting plates having swarf guiding steps, there is also the further disadvantage that the transition between the swarf guiding step and the top of the indexable cutting plate and adjacent the surface machined at any given time is arranged at a fairly great distance from the workpiece surface. Because of this, turnings are able to pass through between the workpiece and the cutting plate, which may lead to damage to the machined surface. Moreover, since the swarf guiding step is not fully formed in the region of the cutting tip, the swarf is frequently not broken and unfavourable lengthy turnings are formed.
The problem underlying the invention is to provide a triangular indexable cutting plate for lathe tools, in particular for precision machining, of the kind mentioned at the beginning with which substantially greater feeds can be achieved, and thereby shorter production times, without impairing the machined workpiece surface, at the same time the life of the cutting edges is also increased and their susceptibility to breakage at the tip is reduced and, moreover, in cutting plates having swarf guiding steps, satisfactory breaking of the swarf is achieved without the risk of the swarf passing through between the surface of the workpiece and the cutting plate.
According to the invention, this problem is solved in that the radius is about 0.15 to 0.3 times the size of the length of the sides of the indexable cutting plate and the centre of this radius is arranged relative to the main cutting edge at a distance towards the centre of the cutting plate and perpendicular to the main cutting edge which correspond to about 0 to 0.3 times the radius.
Therefore, in comparison with known triangular indexable cutting plates, the radius provided at the transition from the main cutting edge to the auxiliary cutting edge is relatively large. While this radius passes over steadily, i.e. tangentially, into the auxiliary cutting edge, there is a tip angle of 90.degree. or a little more at the transition between the radius and the main cutting edge. This relatively large tip angle makes the cutting plate very strong and unsusceptible to breakage at its tip. Surprisingly, it has been found that it is possible to operate with the novel indexable cutting plate at feed rates which are several times higher for the same quality of surface, so that shorter machining times are obtained and thereby also lower machining costs. Furthermore, surprisingly, it has been established that substantial freedom from vibration can be achieved as a result of the comparatively simple features provided according to the invention. In addition, the life of the cutting edges is longer than in the case of the indexable cutting plates known heretofore. Added to this is the fact that with cutting plates having swarf guiding steps a more favourable form and removal of swarf can be obtained. This can be attributed to the fact that the transition from the swarf guiding step to the top of the indexable cutting plate is brought up relatively close to the workpiece surface through the large radius of curvature. As a result, the swarf guiding step is also of optimum form in the region of the cutting tip and the desired bending of the swarf and the breaking thereof is thereby obtained. Turnings are also prevented from being able to pass through between the surface of the workpiece and the auxiliary cutting edge, which is important in particular in the machining of bores. Due to the longer life of the cutting edge, it is also possible to produce greater bore lengths for a constant or almost constant bore diameter, which is a problem in particular in the case of large bore diameters and great bore lengths, since due to the long path of contact between the cutting edge and workpiece in precision turning the cutting edge already wears down so considerably during the machining of a single bore that different bore diameters are obtained at the beginning and end of the bore.